Casting

Finally it is time to cast! Heat enough metal in your ladle for
several castings. Heat it only long enough to melt it. It can
be overheated to the point where it will burn the silicone rubber.
Be especially careful if the silicone's temperature resistance
is at the low end of the acceptable range: 400-450 degrees.

Hot metal will flow best into a mold that is already hot. Warm
the mold in an oven to pre-heat it or warm it by making several
quick pours. Hold the two mold halves together with a wire twist
tie. Do not use rubber bands. They part immediately when touched
by molten metal from drips or spills. The results from these pours
will be discards. Put them right back in your melting pot for
reuse. The nice thing about casting with metal is that mistakes
can be recycled.

Once you are satisfied the Mold is properly pre-heated, make your
pour. Allow it a few minutes to solidify. You will notice a change
in appearance when this happens. Carefully remove the casting
from the mold taking care as it will still be quite hot. If you
are satisfied with the casting, cast more while the mold and metal
are still hot.

If the casting did not fill as expected here are some things to
check:

Are you certain your metal is good quality? Try casting with
that sample of known quality you saved for just that purpose.
Is the funnel opening large enough and in the right place? If
you are getting sufficient quantity of metal into the main cavities
the problem is elsewhere.

Is there a vent problem? If a small detail (example: a bayonet)
does not fill, it is usually due to an insufficient vent. Enlarge
the one already there or add one as necessary. This is not difficult
with an RTV mold from liquid silicone. A sharp knife will cut
it easily. Such is not the case with silicone sealant. It is extremely
tough, as previously mentioned, and does not cut easily. A good
way to add a vent is to cut the silicone part of the mold with
diagonal cutters, then carve the plaster with a knife. If the
vent should happen to be too large or misplaced, silicone sealant
works well for repairing it.

Inevitably your molds will tear or burn out through long or rough
use. Molds with deep undercuts are particularly prone to this.
When this happens do not throw them out! There are uses for old
molds. Chop them up into small pieces and use them as filler for
new molds thereby saving some of that precious liquid RTV. It
is also possible to carve out the inside and use it as the outer
shell of a new mold.